1. Field of the Invention
The present invention relates to controlling the traction motors of a road or rail vehicle having electric traction or electric transmission (as applies to diesel-electric vehicles).
2. Description of the Related Art
At present, electric traction devices are servo-controlled to torque or to current, i.e. the control system seeks permanently to cause the electric motor(s) under its control to provide a torque or to take a current equal to a reference value given to the control system. If the reference value produces a force which is greater than that which can be transmitted to the ground by the mechanical transmission (as happens with reduced adhesion or "grip", e.g. due to bad weather conditions), then wheelspin occurs if traction is being applied, or wheellocking occurs if braking is being applied. At present, in order to overcome these phenomena, an antispin or an antilock system is added to the control system to correct the reference value as provided by a human or an automatic driver so that the control system for the traction device is always given a reference value that is no greater than the torque which the adhesion allows to be transmitted.
This antispin or antilock system which may be implemented in analog electronic form or as software in a microprocessor system, requires the following to be known:
the speeds of the wheels driven by the transmission under consideration;
a reference speed representative of the real speed at which the vehicle is moving and which may either be measured using a wheel on an axle which is under-motorized or not motorized (and which therefore runs no risk of spinning) or which is underbraked or not braked (and which therefore runs no risk of locking), or which is measured by some other device, e.g. using the Doppler effect, or else which is calculated (as described in French patent No. 2 512 556, for example) from the speeds of all of the wheels of the vehicle (integrating a plausible mean acceleration for the vehicle up to a maximum set by the smallest wheel speed in traction or down to a minimum set by the greatest wheel speed in braking). On the basis of this information, the antispin or antilocking system calculates and filters the difference of each speed relative to the reference speed, the acceleration of each wheel, and optionally the derivatives of these accelerations, and it generates a spin or lock signal on the basis of all these magnitudes, which signal is used to correct the reference given by the driver. This correction is performed as follows, as shown in FIG. 1 which shows the input reference CI and the corrected reference CI' as a function of time:
in traction, as soon as the beginning of the wheelspin is detected, i.e. at an instant marked t.sub.0 in FIG. 1, the reference CI.sub.0 that was present at the instant spin appeared is suddenly and considerably reduced, thereby enabling wheels that have lost adhesion to regain it (stage marked T.sub.1 in FIG. 1), and is then raised again quickly to a value which is slightly lower than that which it had at the instant when spinning appeared (stage T.sub.2). Thereafter the applied reference is then further increased slowly up to the value it had when spin appeared (stage T.sub.3), and finally it is increased very slowly until it catches up with the reference value now being input (stage T.sub.4). Naturally this entire procedure is reinitialized each time spinning is detected.
In braking, the reference value is corrected so that the slip of each wheel always limited to a certain value, generally in the range 10% to 20% of the reference speed.
This antispin or antilock technique suffers from the following drawbacks:
It is relatively complex. The processing performed which is described above merely in simplified form, is complicated which means that if performed by analog electronics it requires a large number of circuits thus giving rise to equipment which is voluminous and expensive, and if performed by software integrated in electronic microprocessor control means, then the software is long which may require the size of the memory in the electronic card containing it to be increased, and above all which occupies execution time that can constitute a heavy penalty in a "real time" system.
It requires the speeds of all of the wheels and of all of the axles driven by the traction device to be measured accurately.
It gives rise to severe mechanical stresses on the device mechanics, particularly during traction, because of its repeated, rapid, and major actions taken on the generated torque.
It is a function of the response time of the servo control of the traction system which is downstream therefrom and which responds more or less quickly, thereby requiring numerous adjustments and fine tuning of the system on the vehicle itself.
In traction, by virtue of the theory as described above, it does not guarantee maximum utilization of the available adhesion CI.sub.0, as can be seen from FIG. 2 which also shows the applied reference value CI and the corrected reference value CI' as a function of time:
because of the "holes" in the reference value (shaded zones in FIG. 2) for enabling the wheels to recover adhesion; and
because of the slowness with which the corrected reference value returns to the input reference value after adhesion has become good again (e.g. on entering a tunnel after a zone in the rain).